Glow plugs, particularly for use with Diesel engines and especially for automotive applications frequently have a spiral resistance heating wire which is embedded in a compacted powdery or pulverized mass, for example magnesium oxide. The resistance heating spiral and the magnesium oxide are retained in a thin-walled, closed, sleeve-like metal tube and then compacted so that the spiral heating resistor is tightly retained within the powdery filler in the tube. It has been found, particularly if the resistance wire is made of a material having low ductility, for example tungsten, that the compacting step, done by circumferential hammering or other machining operations, may interfere with the integrity of the cross section of the resistance element; for example, the resistance element may be internally deformed, causing fissures therein, or localized increases in diameter which, in turn, causes a decrease of the electrical resistance value. Resistance wires presently used for most glow plugs customarily contain drawn wire material which, however, also is subject to damage upon being subjected to the compaction step.